Electrophoretic determination of lactate dehydrogenase (LD) isoenzymes in serum samples of surgical patients improved the diagnostic discrimination of myocardial infarction in these patients. The same LD isoenzyme test was used for the determination of cytoplasmic LD isoemzyme patterns in normal and abnormal lymphocytes. Characteristic patterns were established that serve as biochemical markers for differentiation of these cells and aid the diagnosis of lymphoproliferotive diseases.